Commit message: LZWDecodeCompat(): fix potential index-out-of-bounds write. Fixes http://bugzilla.maptools.org/show_bug.cgi?id=2780 / CVE-2018-8905

The fix consists in using the similar code LZWDecode() to validate we
don't write outside of the output buffer.
CVE: CVE-2018-8905
CWE: CWE-787
Vulnerability description: In LibTIFF 4.0.9, a heap-based buffer overflow occurs in the function LZWDecodeCompat in tif_lzw.c via a crafted TIFF file, as demonstrated by tiff2ps.

Project: libtiff
File: tif_lzw.c

LZWDecodeCompat(TIFF* tif, uint8* op0, tmsize_t occ0, uint16 s)
{
	static const char module[] = "LZWDecodeCompat";
	LZWCodecState *sp = DecoderState(tif);
	char *op = (char*) op0;
	long occ = (long) occ0;
	char *tp;
	unsigned char *bp;
	int code, nbits;
	long nextbits, nextdata, nbitsmask;
	code_t *codep, *free_entp, *maxcodep, *oldcodep;

	(void) s;
	assert(sp != NULL);

	/*
	  Fail if value does not fit in long.
	*/
	if ((tmsize_t) occ != occ0)
	        return (0);

	/*
	 * Restart interrupted output operation.
	 */
	if (sp->dec_restart) {
		long residue;

		codep = sp->dec_codep;
		residue = codep->length - sp->dec_restart;
		if (residue > occ) {
			/*
			 * Residue from previous decode is sufficient
			 * to satisfy decode request.  Skip to the
			 * start of the decoded string, place decoded
			 * values in the output buffer, and return.
			 */
			sp->dec_restart += occ;
			do {
				codep = codep->next;
			} while (--residue > occ);
			tp = op + occ;
			do {
				*--tp = codep->value;
				codep = codep->next;
			} while (--occ);
			return (1);
		}
		/*
		 * Residue satisfies only part of the decode request.
		 */
		op += residue;
		occ -= residue;
		tp = op;
		do {
			*--tp = codep->value;
			codep = codep->next;
		} while (--residue);
		sp->dec_restart = 0;
	}

	bp = (unsigned char *)tif->tif_rawcp;
#ifdef LZW_CHECKEOS
	sp->dec_bitsleft = (((uint64)tif->tif_rawcc) << 3);
#endif
	nbits = sp->lzw_nbits;
	nextdata = sp->lzw_nextdata;
	nextbits = sp->lzw_nextbits;
	nbitsmask = sp->dec_nbitsmask;
	oldcodep = sp->dec_oldcodep;
	free_entp = sp->dec_free_entp;
	maxcodep = sp->dec_maxcodep;

	while (occ > 0) {
		NextCode(tif, sp, bp, code, GetNextCodeCompat);
		if (code == CODE_EOI)
			break;
		if (code == CODE_CLEAR) {
			do {
				free_entp = sp->dec_codetab + CODE_FIRST;
				_TIFFmemset(free_entp, 0,
					    (CSIZE - CODE_FIRST) * sizeof (code_t));
				nbits = BITS_MIN;
				nbitsmask = MAXCODE(BITS_MIN);
				maxcodep = sp->dec_codetab + nbitsmask;
				NextCode(tif, sp, bp, code, GetNextCodeCompat);
			} while (code == CODE_CLEAR);	/* consecutive CODE_CLEAR codes */
			if (code == CODE_EOI)
				break;
			if (code > CODE_CLEAR) {
				TIFFErrorExt(tif->tif_clientdata, tif->tif_name,
				"LZWDecode: Corrupted LZW table at scanline %d",
					     tif->tif_row);
				return (0);
			}
			*op++ = (char)code;
			occ--;
			oldcodep = sp->dec_codetab + code;
			continue;
		}
		codep = sp->dec_codetab + code;

		/*
		 * Add the new entry to the code table.
		 */
		if (free_entp < &sp->dec_codetab[0] ||
		    free_entp >= &sp->dec_codetab[CSIZE]) {
			TIFFErrorExt(tif->tif_clientdata, module,
			    "Corrupted LZW table at scanline %d", tif->tif_row);
			return (0);
		}

		free_entp->next = oldcodep;
		if (free_entp->next < &sp->dec_codetab[0] ||
		    free_entp->next >= &sp->dec_codetab[CSIZE]) {
			TIFFErrorExt(tif->tif_clientdata, module,
			    "Corrupted LZW table at scanline %d", tif->tif_row);
			return (0);
		}
		free_entp->firstchar = free_entp->next->firstchar;
		free_entp->length = free_entp->next->length+1;
		free_entp->value = (codep < free_entp) ?
		    codep->firstchar : free_entp->firstchar;
		if (++free_entp > maxcodep) {
			if (++nbits > BITS_MAX)		/* should not happen */
				nbits = BITS_MAX;
			nbitsmask = MAXCODE(nbits);
			maxcodep = sp->dec_codetab + nbitsmask;
		}
		oldcodep = codep;
		if (code >= 256) {
			/*
			 * Code maps to a string, copy string
			 * value to output (written in reverse).
			 */
			if(codep->length == 0) {
				TIFFErrorExt(tif->tif_clientdata, module,
				    "Wrong length of decoded "
				    "string: data probably corrupted at scanline %d",
				    tif->tif_row);
				return (0);
			}
			if (codep->length > occ) {
				/*
				 * String is too long for decode buffer,
				 * locate portion that will fit, copy to
				 * the decode buffer, and setup restart
				 * logic for the next decoding call.
				 */
				sp->dec_codep = codep;
				do {
					codep = codep->next;
				} while (codep->length > occ);
				sp->dec_restart = occ;
				tp = op + occ;
				do  {
					*--tp = codep->value;
					codep = codep->next;
				}  while (--occ);
				break;
			}
			assert(occ >= codep->length);
			op += codep->length;
			occ -= codep->length;
			tp = op;
			do {
				*--tp = codep->value;
			} while( (codep = codep->next) != NULL );
		} else {
			*op++ = (char)code;
			occ--;
		}
	}

	tif->tif_rawcc -= (tmsize_t)( (uint8*) bp - tif->tif_rawcp );
	tif->tif_rawcp = (uint8*) bp;
	sp->lzw_nbits = (unsigned short)nbits;
	sp->lzw_nextdata = nextdata;
	sp->lzw_nextbits = nextbits;
	sp->dec_nbitsmask = nbitsmask;
	sp->dec_oldcodep = oldcodep;
	sp->dec_free_entp = free_entp;
	sp->dec_maxcodep = maxcodep;

	if (occ > 0) {
#if defined(__WIN32__) && (defined(_MSC_VER) || defined(__MINGW32__))
		TIFFErrorExt(tif->tif_clientdata, module,
			"Not enough data at scanline %d (short %I64d bytes)",
			     tif->tif_row, (unsigned __int64) occ);
#else
		TIFFErrorExt(tif->tif_clientdata, module,
			"Not enough data at scanline %d (short %llu bytes)",
			     tif->tif_row, (unsigned long long) occ);
#endif
		return (0);
	}
	return (1);
}